1. Field of the Invention
This invention relates to metallized plastic film products and their use in composite structures. In a preferred embodiment, this invention relates to electrically conductive films and their use in shaped composite structures.
2. Description of Prior Art
It has long been recognized that metallic layers can be deposited on plastic films. Chemical vapor deposition techniques, plating processes, magnetron sputtering and ion sputtering are representative processes for depositing nanometer thick (or thicker) films of metals, metal compounds, and the like on plastic materials such as plastic sheets. A typical product produced heretofore is a plastic body or a plastic film having a metal layer on one or both sides. The usual film product is a continuous sheet product. These sheet products find applications in electronic devices, in windows as light reflectors, and in other applications where a thin sheet of metal is useful.
In these and many other applications, it may be desired to incorporate the metallized plastic sheet material into laminated composite structures. In these structures, the metallized sheet can provide an electrically conductive surface or impart other useful metal properties to the composite. While conventional metallized plastic sheet products have been used in these applications, they have two inherent characteristics which interfere with their acceptability in high performance areas.
The first of these two problems is that the metallized plastic layers, being continuous plastic sheets, have difficulty conforming to complex or compound curves. This can be solved in some cases by tailoring the sheet to fit the object to which it is being laminated or by folding the sheet to conform. So too, a stretchable continuous substrate can also give some degree of conformance to the complex compound curves. However, these solutions are far from ideal. The cutting and folding approaches can be extremely labor intensive. They also can introduce or give rise to breaks in the film's metal coating which can degrade or destroy desired electrical properties. The use of stretchable substrates presents the problem that the metal coating often is less able to stretch than the substrate and thus breaks or separates from the substrate.
The second problem relates to the strength of the adhesion between either the metal layer and the substrate, the composite resin and the metal layer, or the composite resin and the plastic substrate. Often, the grip between these surfaces is relatively poor. This can be a significant problem when the material is incorporated into laminates because the boundary between the plastic sheet or the metal coated on the sheet and the surrounding composite resin can serve as shear planes which can disrupt or defeat the strength of the overall laminate structure.
One way to address this second problem would be to improve the adhesion of the metal layer to the substrate. Drauglis et al, in USP 4,374,717, issued in 1983, suggest that the adhesion of sputtered chromium to urethane substrates can be improved by applying a coating of plasma polymerized acetonitrile under and over the metal layer. Manabe, et al, in USP 4,369,225, issued in 1983, suggest that an improved metal coating is achieved when the metal is undercoated with a flexible urethane paint and then overcoated with a similar material. Gliem, et al, suggest in USP 4,364,792, issued in 1982, that the adhesion of metals can be improved by etching the surface of the polymer to roughen it prior to coating. Kumagai, et al, show another approach in USP 4,402,998, issued in 1983, when they suggest that a primer coat of metal be laid down, followed by a catalyst layer and finally the desired metal layer. All of these representative disclosures point up the problem with adhesion. Their solutions may work in certain low performance areas, such as decorative coatings and sheltered electronic devices, but they do not suggest that their solutions are adequate in high performance areas where the shear plane can severely detract from the strength of ultrahigh strength composite materials as are found in aircraft and the like.
When we produced metallized thin films for incorporation into such high performance composite structures, the above-described failings appeared. It is, therefore, an object of this invention to solve these problems presented by prior metallized plastic films.